This invention relates to a process for making porous anodes for electrolytic capacitors. The capacitors may be either of the sophisticated type and the solid type.
Anode bodies for use in manufacturing these anodes have been made by adding a binder, such as water, camphor, or polyethylene glycol, to metal powder for the anodes, such as tantalum, niobium, titanium, or aluminium powder, weighing the resulting mixture in compliance with the electrostatic capacity for each anode, press-forming pellets cavities of the mixture together with lead wires for the anodes in cavities of the mold, and sintering the pellets into anode bodies each having a lead wire. Subsequently, solid electrolytic capacitors may be formed through the steps generally called the batch assembly, by arranging the anode bodies in order and with a predetermined orientation, welding a metal support to the lead wires, anodizing the anode bodies by application thereto of an electric power of a predetermined current until the voltage reaches a predetermined value in an anodizing solution, such as an aqueous solution of sulfuric or phosphoric acid, to form dielectric oxide layers on the respective anode bodies, depositing semiconductor layers, such as manganese dioxide layers, on the dielectric layers, further depositing conductive layers, such as colloidal carbon and silver paint layers, on the semiconductor layers, and putting the capacitor elements thus formed in capacitor casings.
On press-forming the pellets, it is necessary to make the weighed mixture flow into each mole cavity through a hopper in a short period of time. In addition, it is necessary to press-form each pellet. This complicates the process and requires that the flowability of the mixture and the mechanical strength of the pellets be sufficient to avoid deformation before sintering thereof. Therefore, particular care must be paid to preparation of the mixture and further complicates the process. Even if specific care is taken, it is necessary to press-form the mixture into pellets of a high bulk density to avoid deformation. This reduces the porosity of the anodes and the dimensions of the pores formed therein, thereby reducing the effective surface area of each anode and the electrostatic capacity per unit weight of the anode while increasing the loss factor (tan .delta.) of the capacitor and adversely affecting impregnation of the anodes with the semiconductor material.
In U.S. Pat. No. 3,467,557 issued to J. B. Fincham, a process for making an electrolytic capacitor porous anode is disclosed which comprises in essence the steps of introducing metal powder for the anode into a mold to provide a loosely disposed mass of the powder with a bulk density of about 3 to 6 g/cc, pre-sintering the powder to form a coherent self-supporting pellet, and sintering the pellet into the anode. This requires an additional step of pre-sintering.
In U.S. Pat. No. 3,422,515 issued to G. P. Klein, a process of the kind described is revealed which comprises in essence the steps of moistening metal powder for the anodes, placing the moistened powder in mold cavities, freezing the powder into frozen pellets, thawing the frozen pellets, and sintering the resulting powder deposits. This requires additional steps of freezing and thawing.
With both of these processes of the patents, it is possible with advantages to reduce the bulk density of the pellets. The pellets, however, are formed in the respective mold cavities. This requires no less flowability of the metal powder than the sophisticated processes and no less complicated steps of preparing the metal powder. In addition, the additional processes mentioned above are necessary in order to achieve sufficient mechanical strength of the pellets.